The present invention relates to band gap reference circuits, and more particularly, to band gap reference circuits that maintain a constant output voltage over a range of temperature and bias current.
A band gap reference circuit provides a constant output reference voltage VREF. Problems may arise if the output reference voltage VREF varies even by a small amount such as a few hundred millivolts over a range of temperature or bias current. Therefore, it is desirable to provide a band gap reference circuit that provides an output reference voltage VREF that is substantially constant over a range of temperature and bias current.
Previously known standard CMOS band gap reference circuits typically include an amplifier that comprises a differential pair of p-channel MOS transistors. VREF is determined by the voltage at the gate of one of the p-channel MOS transistors. Excess charge carriers can become trapped in the silicon to silicon dioxide (SiO2) interface in MOS transistors. The excess charge may cause variations in the threshold voltages of the MOS transistors in the differential pair of the amplifier. For example, the threshold voltages of the two MOS transistors in the differential pair may differ by more than 5 mV. This difference introduces an offset voltage into the amplifier which appears at VREF of the band gap reference circuit. The offset voltage can prevent the band gap reference circuit from being adjusted with trimming resistors so that VREF remains constant with temperature changes.
In addition, the charge trapped in the silicon/SiO2 interface of the differential pair MOS transistors in the band gap reference amplifier can vary over time causing VREF to change over time even at a constant temperature. These variations in VREF cause undesirable 1/f output noise. Also, the p-channel MOS transistors in the differential pair may introduce thermal noise at VREF due to the nature of MOS transistors, which is also undesirable.
A further disadvantage of previously known standard CMOS band gap reference circuits is that they are sensitive to relatively small changes in the supply voltage VCC. Small changes in VCC cause variations in the bias current through the band gap reference circuit, which can cause undesirable changes in VREF.
It would therefore be desirable to provide a less noisy band gap reference circuit in CMOS technology that provides a substantially constant output reference voltage VREF over a range of supply voltage and a range of temperature.
The present invention provides CMOS low noise band gap reference circuits that output a substantially constant reference voltage VREF. Band gap reference circuits of the present invention have an amplifier that includes a differential pair of bipolar junction transistors. Each of the bipolar junction transistors are coupled to a first or a second plurality of bipolar junction transistors or a first and second plurality of diodes. The first and second plurality of transistors or diodes are coupled to a plurality of resistors. When the temperature of the circuit varies over a range, the change in the voltage drop across the resistors compensates for the change in the voltage drop across the transistors or the diodes so that VREF remains substantially constant.
A feedback circuit is coupled to the amplifier. The feedback circuit adjusts its current to compensate for variations in the supply current so that the VREF remains substantially constant. The band gap reference circuits of the present invention provide a output reference voltage VREF that is substantially constant with variations over a range of temperature and supply voltage. Band gap reference circuits of the present invention may be fabricated using standard CMOS process techniques.